Clay liners are widely used in the fields of Civil, Environmental and Geotechnical Engineering for waterproofing surfaces of soil, lagoons, foundation surfaces, and in particular constructions for the containment of pollutant or toxic waste. The known clay liners are generally formed by a layer of swellable clay between two fabric layers (geotextiles). The clay used in this type of liner is predominantly Sodium Bentonite, which, when it comes into contact with water or other liquids, will swell to several times its dry volume if not restrained by a confining stress. The swelling of the clay forms an impermeable barrier. The clay is usually confined between two fabrics and the clay and fabrics are held together by either bonding with water soluble adhesive or mechanical bonding.
In adhesive bonding (see U.S. Pat. No. 4,501,788) the two fabrics and the clay are held together by a glue which permeates through the clay holding the particles together and also holding the fabrics to either side of the clay layer. With mechanical bonding the clay is in dry form either powder or granular and is held loosely in position between fabrics by needlepunching the two fibres together (see EP-A-278419). Needlepunching is a form of bonding using barbed needles which drag the fine fibre threads from one fabric to the other through the layer of clay thus holding together the two fabrics with the loose clay between. The fibres dragged across are tangled with the fibres in the main body of the textile and not fastened in any way. The process is accomplished on a machine that uses two needle boards one on the top and one on the bottom. These boards contain hundreds of barbed needles and move up and down very quickly driving the nest of needles through the liner as it passes between.
Like all clays when it is hydrated, which it must be to maintain its impermeability, it becomes very slippery and therefore unstable on a steep slope. To overcome the instability, due to a shear strength failure through the clay layer, the fabrics on either side of the clay layer must be connected to one another in some way through the clay layer.
The fine fibres mentioned in the needlepunched material also increase the shear strength of the clay liner although this is not their primary function. Needlepunched material has a number of disadvantages. The strength of the fine fibres connecting the two fabrics is low unless the intensity of the needlepunching is increased considerably, and if this is done the increased needlepunching destroys the impermeability of the liner. A further problem with the needlepunching process is that a nonwoven fabric must be used on at least one side of the liner to obtain the fine fibres for the bonding. The nonwoven material allows a lateral flow of the liquid to be contained through the plane of this material giving a problem with leakage at the edge, or overlap, of the liner. If a woven material is used, which is preferable in this type of liner, the weave would be damaged or destroyed by the barbed needles used in the process of needlepunching and weaken the liner.
In using this type of needlepunched liner on steep slopes there is also a problem with the movement of bentonite clay within the liner. The clay in its dry form, either powder or granules, is free to move and in the process of handling during loading, unloading and installation, tends to migrate causing light areas with little or no clay and potential leakage in the depleted area. This can also happen when the clay is in a state of hydration. When the clay is hydrated without cover material to form the confining stress, the hydrated bentonite becomes very swollen and soft, if then the cover material is placed on the liner it will displace the hydrated clay causing an increase in permeability and the liner to leak.
Another problem encountered with needlepunched clay liners is that they cannot be used in the field with a simple overlap. The seams must be sealed with a dry bentonite clay powder or a bentonite paste which is placed in position by hand. This is a very labor intensive operation, time consuming and costly. It does not always seal the seam and depends on proper supervision and human error. If two nonwoven textiles are used this particular problem is increased. It is very difficult for the labour force to apply the seam sealing bentonite on steep slopes.
A sewn liner which is the only other mechanically bonded clay liner in use, is disclosed in EP0490529. It consists of a glue bonded material with lines of stitching through the material in the machine direction to give a higher friction angle. The stitches are lock stitches requiring one thread on top through the needle which then passes the thread through the liner, while a second thread on the bottom of the liner, supplied by a separate spool, locks the top thread. The machinery to do this sewing is very expensive and the sewing can only be done in a straight line as it is impractical to form a pattern or deviate from the straight line (too slow) and cannot be carried out with this machinery. Furthermore, this type of liner is subject to some problems manifest in the needlepunched liner. The glue bonded liner which is then sewn to cope with a steep slope situation, is initially bonded with a water soluble glue to bond the fabrics of the liner and also the clay particles together. The glue used in this process when dry becomes very brittle and the process of sewing the two fabrics together tends to break up the dried glue and cause the bentonite granules to become loose. The bentonite is then free to move within the confines of the linear sewing of the liner. This sewing is only in the machine direction and runs parallel the full length of the liner, the lines of sewing being some 100 mm apart. The loose bentonite material can migrate a considerable distance in the machine direction again causing areas of limited bentonite and a potential failure of the liner. The soluble adhesive, being brittle when dry, can also cause this problem during the handling of the sewn clay liner, when the liner is being loaded, unloaded and during installation on steep slopes where the dry loose bentonite will move down the slope in the machine direction between the lines of stitching which run in the same machine direction vertically up and down the slope.
The parallel stitching in the machine direction at 100 mm apart is the only practical way in which this type of mechanical bonding can be carried out. Although the distance between the rows of stitches can be varied, the stitches themselves can only run in a straight line in the direction of production; this means that this problem is actually without a satisfactory solution.
The same problem applies to the sewn liner when the bentonite is hydrated, as with the needlepunched liner, if any machinery or pedestrian traffic or cover material is placed on the liner. In its hydrated form the bentonite will be free to move in the liner within the lines of stitching causing an area of potential leakage where the bentonite is displaced. A lesser problem is the weight of the cover soil or other material which, pulling down the slope, will tend to increase the tensile forces on the cross weave of the top fabric of the clay liner which is the weaker weave of the liner. This is due to the stitching being in line with the forced down the slope and not being staggered or offset to distribute the load or force acting in the machine direction.
EP-0611850 discloses a further type of geosynthetic clay liner including a layer of bentonite disposed between a carrier sheet and a cover sheet which are linked to one another via a plurality of tufting threads formed by a conventional tufting machine. A bight or loop portion of the tufting thread extends through to the undersurface of the carrier sheet and is partially melted to prevent the bight portion from slipping back through the carrier sheet. Due to the large number of tufting threads crossing the bentonite layer there is no need for gluing the bentonite to the inner side of the carrier sheet and the cover sheet. The partially melted bight portions create an undersurface with a high coefficient of friction thereby improving the linear stability on steep slopes. However, the liner according to this disclosure is provided with fastenings arranged on parallel or staggered lines only and has an open pattern of the fastenings which causes the bentonite to be displaced under heavy stresses or by gravity when the clay is either dry or hydrated.
Furthermore, in this liner the presence of a plurality of tufts (i.e. about 15 tufts or loops per inch as usual in tufting machines) not only increase the permeability of the liner, but also weakens the fabric, thereby the liner is unable to withstand multidirectional high shear stress. The production process of this type of geosynthetic clay liner is also complicated by the fact that the liner must be sewn in correspondence to its edges before being passed through the tufting machine.
Bentonite liners are used on the steep slopes of landfills to protect the environment by forming an impermeable barrier on the slope. The steep slopes enable the landfill to be used more efficiently so that the landfill will have a larger containment volume. This is also true of other installations such as tank farms which have steep slopes on the secondary containment areas in case of the tanks rupturing. The bentonite liners therefore need to have higher shear strength, and the bentonite to be so confined that the forces acting down the slope will not cause the liner to fail mechanically or the bentonite to become unstable when used on these slopes.
It is an object of the present invention to provide a flexible clay liner to form a impermeable barrier having a higher stability of the clay layer and higher shear strength with respect to the prior art clay liners, as well as stability on steep slopes and in standing water or inclement weather conditions.
It is another object of the present invention to provide a method for mechanically bonding a geosynthetic clay liner of the above mentioned type which allows the formation of independent fastening means for connecting the fabrics containing the clay liner arranged according to a closed pattern to confine the clay particles.
It is a further object of the present invention to provide a method for mechanically bonding a geosynthetic clay liner of the above mentioned type providing a liner with a surface coarseness adjustable on one or both liner sides according to the needs.
Still another object of the present invention is to provide a machine for mechanically bonding a geosynthetic clay liner of the above mentioned type operating according to the method of the invention.
The above objects are accomplished with the clay liner according to the invention consisting of two woven geotextiles both of high tensile strength in the machine and cross directions, a semihydrated layer of powdered or granular swellable clay between said geotextiles and fastening means connecting the two geotextiles and arranged in such a pattern of connecting threads to form pockets through the clay liner. The fastening means comprises rows of independent fasteners arranged according two intersecting directions, forming an angle with the longitudinal axis of the liner. Each fastener comprises at least a pair of sections of thread passing through the clay liner and the geotextiles and protruding therefrom to form a small loop on at least the underside of the liner, said loop being fixed and sealed to the geotextile surface or, preferably, melted into it to form a knot at each fastening site.
The method of the invention comprises the steps of:
preparing a geosynthetic clay liner, comprising two woven geotextiles both of high tensile strength in the machine and cross directions and a semihydrated layer of powdered or granular swellable clay between said geotextiles, according to known methods;
forming on the geosynthetic clay liner symmetrically alternated longitudinal seams having a substantially zigzag path, each seam meeting the adjacent one at least in correspondence of their edges or crests and being formed by threads passing through the liner in correspondence of each fastening site to form a closed pattern;
forming a protruding loop of thread at each fastening site on at least one side of the liner;
locking the so formed loops to prevent them from unthreading.
As a preferred embodiment of the method of the invention, loops are formed on both sides of the liner and are then melted into the geotextiles to form protruding knots on the independent fasteners.
The machine for producing a geosynthetic clay liner according to the invention and operating according to the above described method comprises a fixed machine frame, means for moving the liner along a feeding direction, a pair of parallel needle bars each carrying at least a row of needles and arranged crosswise with respect to the feeding direction and axially reciprocating in opposite direction with respect to each other, said needle bars further being reciprocated in a direction perpendicular to the plane along with the liner is moved through the machine so as to form corresponding seams extending in the feeding direction with a substantially zigzag path. The machine further comprises a looper bar parallel to the pair of needle bars and placed on the opposite side to them with respect to the liner, from the looper bar there extending radially looper plates in a side-by-side spaced relationship, the looper bar having a rocking movement to engage each looper plate with the thread when the needle has gone to the end of its downstroke thus forming the loop. Means are provided for locking the loop and sealing the passage hole for the thread through the geotextile and means for moving in synchronism said needle bars and said looper arm.
In a preferred embodiment of the machine the means for locking the loops and sealing the corresponding passage holes comprises singeing means for melting the loops and forming protruding knots therefrom. In a further preferred embodiment a needle plate with a plurality of side-by-side spaced apart fingers is provided in a parallel position with respect to the needle bars and from the same side of them with respect to the liner. The needle plate is reciprocated axially in synchronism with the needle bars so that each finger is alternatively placed at one side and at the other side of a corresponding needle while the latter is at the end of its upstroke. In this way at each fastening site loops can also be formed on the other side of the liner.